The invention relates to an information carrier comprising runlength limited marks in a track, the runlengths of the marks representing main channel bits and variations of a further parameter of the marks representing secondary channel bits.
The invention further relates to a device for encoding, a method for encoding, a device for decoding and a method for decoding.
The invention is applicable to record carriers with different kinds of channel codes. In a channel code source bits are encoded to channel bits according to a predetermined scheme. Information may be stored on record carriers channel coded, for example, in accordance with a runlength-limited (RLL) code. A RLL code is characterized by two parameters, (d+1) and (k+1), which stipulate the minimum and maximum runlengths, respectively, that may occur in the code. The length of time usually expressed in channel bits between consecutive transitions from type of mark is known as the runlength. Such a transition can for example be a transition from a pit-mark to a land-mark, e.g. in CD-DA, CD-R, or a transition from an amorphous domain to a crystalline domain, e.g. in CD-RW.
An information carrier mentioned in the foregoing is known from European Patent Application EP 0 866 454 A2. This document discloses an optical recording medium in which a run length limited channel code is recorded in the form of a mark. The information encoded in this channel code comprises main data and encryption data, encryption data being represented by a width of the mark. During read-out different levels of the read signal have to be detected. This encryption data can not be detected in a very reliable manner.
The invention has for an object to create a more reliable secondary channel, the secondary channel being related to a main channel.
The information carrier in accordance with the invention is characterized namely in that only marks of at least a predetermined runlength have said variations. In this invention, the secondary channel is made reliable in the manner on which the secondary channel is related to the main channel.
The invention is based on the recognition that the reliability of a secondary channel is not equal for all runlengths for which this secondary channel is added to the main channel using multi-level coding.
Multi-level coding can be achieved in different ways. With multi-level coding is meant the coding which uses the different levels of the read-out signal, obtained when reading out a record carrier, to decode data stored on the record carrier. These different levels of the read-out signal can for example be accomplished by changing the geometry of a pit or mark to be read out from the record carrier. This change in geometry can be of different kinds, variation of the width, depth, the number of width or depth variations etc. A physical parameter of the secondary channel can be used for multi-level coding, e.g. a so-called xe2x80x9cpeanutxe2x80x9d-structure can be made, or the depth and/or width of the pits and marks can be varied. The main channel is a binary channel where pits and non-pits (lands) are related to two possible signal levels (below and above a threshold level).
The parameter nmin, which indicates this minimal runlength for which the secondary channel is created, is chosen to be such that normal timing recovery in the main channel is not affected. The secondary channel is hierarchically dependent on the main channel, since secondary channel bits can only be accommodated at those locations in the channel bit stream, where the main channel coding uses longer runlengths. This secondary channel is therefore called to be realized via limited multi-level (LML) coding. The limitation consists of the choice that multi-level coding is only applied for a predetermined minimum runlength.
Using this LML coding technique constitutes advantages. In general, writing short runlengths is more difficult. Experiments have shown that the jitter in the main channel due to pit-land modulation, e.g. modulating the width of a pit, increases for shorter runlengths. Due to this increase in jitter, the read-out reliability of the main channel decreases. As the read-out signal is situated closer to the normal slicer level, the chance of misdetecting such a short runlength is increased. As said before, for short runlengths the equalized eye-pattern is not saturated. In order to be able to detect and reconstruct the secondary channel bit from short runlengths it is therefore necessary to use more slicer levels than when only long runlengths are used for storing the secondary channel bits. These difficulties are overcome if multi-level coding is only applied for a predetermined minimum runlength.
By creating the secondary channel extra capacity can be generated on top of the capacity of the main channel. When reading out a record carrier comprising a secondary channel a traditional player can only see the information stored in the main channel, while a enhanced player, equipped with means for reading out and decoding the secondary channel, can also see the information stored in the secondary channel.
An additional advantage is that the data capacity of a record carrier can be increased by creating the secondary channel.
Another information carrier in accordance with the invention is characterized in that the parameter is the width of the mark or space.
Another information carrier in accordance with the invention is characterized in that the runlength limited data obeys constraints d=2, k=10, and in that the predetermined minimum runlength is 6.
As said before, the minimal runlength for which the secondary channel is created is chosen to be such that normal timing recovery in the main channel is not affected. For example, for DVD, a reasonable value for nmin is 6, since the equalized eye-pattern under DVD-conditions reaches saturation (i.e. maximum amplitude level for land-marks and minimum amplitude for pit-marks) already for 16-runlengths.
Another information carrier in accordance with the invention is characterized in that the secondary channel of data also comprises error-correction data.
Another information carrier in accordance with the invention is characterized in that the marks are pits and lands, the pits having a reduced depth, the lands having a pit of small depth.
As said before, multi-level coding can be performed in different ways. For example, the pits and lands can be mastered in a so-called xe2x80x9cpeanutxe2x80x9d-structure which is realized by turning off the laser at a predetermined place and for a predetermined time in the case of a pit and by turning on the laser at a predetermined place and for a predetermined time in the case of a land. In this way, the pits will have a reduced depth and the lands will have a pit of a small depth.
The device for encoding in accordance with the invention comprises converting means for converting a binary source signal into a channel signal, the channel signal comprising a main channel signal representing main channel bits and a secondary channel signal representing secondary channel bits, the converting means being further arranged for detecting the runlengths of the main channel bits and the converting means further comprising inserting means for generating the secondary channel signal only upon detecting a runlength of a predetermined minimum runlength.
The method for encoding in accordance with the invention comprises the step of converting a binary source signal into a channel signal, the channel signal comprising a main channel signal representing main channel bits and a secondary channel signal representing secondary channel bits, the method further comprising the step of detecting the runlengths of the main channel bits and the method further comprising the step of generating the secondary channel signal only upon detecting a runlength of a predetermined minimum runlength.
The device for decoding in accordance with the invention comprises decoding means for decoding a channel signal into a binary source signal, the channel signal comprising a main channel signal comprising main channel bits and a secondary channel signal comprising secondary channel bits, the device further comprising detecting means for detecting the runlengths of the main channel bits, the decoding means being further arranged for detecting the secondary channel only upon detecting a runlength of a predetermined minimum runlength.
The method for decoding in accordance with the invention comprises the step of decoding a channel signal into a binary source signal, the channel signal comprising a main channel signal comprising main channel bits and a secondary channel signal comprising secondary channel bits, the method further comprising the step of detecting the runlengths of the main channel bits and the method further comprising the step of detecting the secondary channel only upon detecting a runlength of a predetermined minimum runlength.